My dentist crowned tooth 29 4 months ago and did not do a root canal. I am now in pain. My dentist told me that I have an infection and prescribed CLINDAMYCIN 150mg and has scheduled me for a root canal on the crowned tooth. My am wondering why this has happened to a newly crowned tooth. Thank you

A crown without a root canal isn't uncommon...

As far as the infection, it has nothing to do with whether the crown is new. You could have gotten it the day you got the crown, it could have happened 10 years afterwards.

ISneezeFunny is right. Teeth don't like to be worked on too much, if they could talk, they'd say "fix me right the first time". The more often a tooth is worked on it puts the nerve at risk of dying which can cause this kind of infection. Unfortunately, it does happen and a hole has to be drilled through the new crown in order to do the root canal. This will not put the new crown at risk, it will not need to be replaced.